Monitoring the concentration of exhaled carbon dioxide in order to assess the physiologic status of patients with acute respiratory problems or who are receiving mechanical ventilation, and to determine the adequacy of ventilation in anaesthetised patients, is known as Capnography. Two methods are typically employed, Mainstream Capnography and Side-stream Capnography.
The first method, known as Mainstream Capnography, consists of using an appropriate measuring sensor which is coupled and positioned on an appropriate adapter placed on the patient airway tube, connecting-the patient to the ventilation machine. This method has a disadvantage that it requires-that heavier and more cumbersome equipment be positioned in close proximity to the patient, possibly interfering with other vital medical equipment.
A second method, known as Side-stream Capnography, is based on continuous sample of breath being transferred to a remote sensor for measurement and analysis. In this method, the breath sample is transferred via an airway adapter incorporating a breath sampling port positioned in the Patient airway tubing, through a narrow diameter flexible tube, referred to as a breath sampling tube, towards the measuring sensor, known as a Capnograph, for monitoring and analysis. A liquid filtering system is normally used for controlling and handling liquids encountered in the medical environment such as may be related to patient secretions, condensed out liquids resulting from high humidity in the ventilation means, and medications and saline solutions provided to the patient during lavage, suction and nebulization procedures, and which accumulate in the Patient airway tubing.
Capnograph accuracy in monitoring and analyzing a breath sample requires that there be a continuous, smooth, laminar flow in the exhaled gases when moved from the patient to the measuring sensor, such that the effect on the exhaled gas waveform is maintained to a minimum. In a typical side-stream system, a standard airway adapter for breath sampling in a Patient airway tubing-would have as a basic design a tube of approximately 15 mm internal diameter and some 60 mm long with appropriate medical standard conical fittings at either end (to serially connect to the Patient airway tubing). Mid way along the tube would be attached a small bore sampling tube, 1 mm-2 mm internal diameter, referred to as a sampling port, perpendicular-to the axis of the gas flow in the Patient airway tubing, and reaching close to the center-of the tube on one side and on the other side exiting the tube wall to be connected to the breath sampling tube. The Capnograph will then continuously provide a suction effect, typically by means of a diaphragm pump, which is measured as a pressure drop, at the sampling port in order to receive a continuous sample of gas for analysis.
Filtering systems utilized in Side-stream Capnography are designed to attempt to minimize the possibility that liquids can collect in the breath sampling tube and reach the sensitive components of the Capnograph and/or block the breath sampling tube, consequently interfering with the continuous monitoring of the subjects breath. Several methods have been developed for this purpose, some designed to prevent liquids from entering the breath sampling tube, and some designed to remove the liquids once inside the breath sampling tube. These may include:    a. A specially designed airway adapter fitted on the Patient airway tubing and to which is connected the breath sampling tube.    b. A Nafion (or similar) tube, which may form part of the breath sampling tube, for reducing humidity and consequently reducing the possibility for condensed out liquids to collect.    c. A reservoir in the breath sampling tube for collecting liquids accumulated in the breath sampling tube and removing the liquids from the gas flow path.    d. A hydrophobic membrane to prevent any liquid managing to pass the above elements and which will cause damage to the Capnograph.
One of the major obstacles when designing a filter system is the necessity to prevent any liquids from blocking the breath sampling path or from reaching the measurement sensor while providing continuous, smooth, undisturbed sampling of the patients breath (exhaled waveform). This waveform related requirement places a series of constraints on the design of the filter. As described in U.S. Pat. No. 5,657,750, distortion to smooth unimpaired gas flow in the prior art fluid filters are caused by three major factors:    a. By the materials of the filtering device itself, including the porous portions of the filtering membrane and porous portions of the wall thereof, to an extent proportional to their thickness;    b. By the shape or configuration of the filter body itself, preventing abrupt changes in the gas passageway between inlet and outlet thereof, and    c. By the overall size of the volume or space of the passageway for the gas flow from the inlet to the outlet of the filter.
A major problem with currently known filter systems is that typically, the volume of the traps or reservoirs is small, 100-300 micro-liter, so as to have minimum influence on the waveform of the exhaled gas. Hence small quantities of liquid can result in quick overfilling of the reservoir which can then result in blockage of the breath sampling tube.
Many techniques have been used in the attempt to filter out liquids from the exhaled gas without affecting the waveform in the breath sampling tube. Such techniques have included absorbents for wicking out condensate out of the gas, centrifugal filters, dessicants, hydrophobic membranes for filtering gases, and hydrophilic membranes for filtering liquids, all having their advantages and disadvantages. Nevertheless, none are able to remove all the liquids in the breath sampling tube without the intervention of a hydrophobic filter, which may have an effect on the waveform of the exhaled gas. A hydrophobic filter is dictated, since after any reservoir fills, the liquids may overflow and reach-the gas analyzer, hence a hydrophobic filter is used as a fuse together with the liquid reservoir. However, when placing this fuse in close proximity to the liquid reservoir/collector, as typically done, the entry of liquid may often be too fast for the liquid reservoir to absorb the entire volume and part of the liquid may pass the reservoir and reaches the fuse far before the reservoir is full. The design of the reservoir may thus interfere with the waveform, specifically, since the gas sample flows and mixes in the reservoir.
There is therefore a need for a filter system which includes a liquid storage element, more specifically a reservoir, which can be added between the airway adapter and the measurement sensor, and will serve to temporarily store all liquids in such a way that they are dried up over a period of time or dispensed off in small quantities (droplets) along the breath sampling tube such that they also are dried up over time, and which will not disturb the waveform of the exhaled gas.